These pages comprise the efforts of Terry Toohill to explain our human origins through the perspective of evolution. Each post deals with specific aspects of the factors that have influenced not only our own evolution, but those that have driven the evolution of all life on planet earth, from its humble origins, billions of years ago.

Saturday, 20 June 2009

Human Evolution On Trial - Species Or Not?

Species Or Not

The fossil evidence is usually interpreted as showing that around half a million years ago, throughout much of the world, Homo erectus started to evolve into Homo heidelbergensis. It may be significant that mitochondrial DNA studies indicate the ancestors of Neanderthals and modern humans also separated about that time (Wade 2001). Homo heidelbergensis is usually said to have followed Homo erectus out of Africa carrying the newly developed Levallois method of making stone tools.

Homo heidelbergensis

Homo heidelbergensis is a name for a kind of “Archaic Homo sapiens” but it is doubtful if it should be considered on the one hand to be a single species or on the other a separate species from either Homo erectus or Homo sapiens. For example Richard Klein (1989) states that during this period once again it is often difficult to decide which species a particular fossil should be classified as. That’s why Homo heidelbergensis might be a species that’s not a species. It’s simply a species or series of species placed between Homo erectus and Homo sapiens to emphasise the difference between these two extremes in time. Other species in the human line are probably also in this situation. Perhaps humans have actually changed gradually over both time and space. Precise boundaries between the different species through time and space are impossible to define. But populations at the extremities of any species’ range are always the most different. The name Homo heidelbergensis is a useful name to keep though. It fits with the change in appearance and technology that occurred between about half and quarter of a million years ago. Besides, it’s a nice name.

Open grassland with trees scattered through it probably remained the Homo heidelbergensis preferred, and most easily exploitable, environment (“First Humans” [Homo erectus]). If so Homo heidelbergensis would have moved through this environment before exploiting less desirable locations (“Eastern Polynesia”[Marginal Polynesia]). But they were able to colonise only some of the region Homo erectus had occupied. Single origin supporters presumably believe Homo heidelbergensis replaced Homo erectus in any areas they managed to get to as, in fact, they may have done in the Middle East and Europe (see “Technology”[Middle Palaeolithic]).

Of course this is quite likely to have been because the region’s earlier inhabitants had already become extinct for one reason or another. This could have been because of severe climatic drying through Central Asia, Arabia and North Africa. Homo erectus may have died out through the middle of the human star. The human populations in the points became isolated and some, such as Homo antecessor in Europe, may even have died out. With the return of good conditions various populations could expand back through any unoccupied regions or ecological niches. If the spread origin people are correct the various species formed hybrids in many places. The genes mixed.

East Asian Point

Evidence from Northern China at Zhoukoudian Cave shows that nearly half a million years ago Homo erectus in the East Asian point of the human star was already developing into the modern East Asian type. A lower jaw dated at between half a million to 300,000 years ago lacks the third molar. This is a characteristic of about 30% of people of East Asian origin today (Bellwood 1975). There is also the problem of what are called shovel-shaped incisors found in a high proportion of both ancient and modern eastern populations.

It is hard to believe that if all humans are descended solely from a more recent migration from Africa these two characteristics could have arisen again independently in the same place twice. Both these characteristics are also found in European Neanderthal and some other early human fossils but many people also believe the general facial features of modern Asians, such as delicate, flattened faces, were already present in ancient times. Incidentally this would undermine claims humans arrived in America before the modern East Asian human type had evolved (“North to Alaska”[First Americans]).

Some human skulls found in North China dating from about 350,000 years ago show that Homo erectus populations there were definitely already moving towards the modern human type by then. This is long before any “single origin” people could have reached the region. The skulls have much bigger brain cases than do earlier Homo erectus skulls (Stringer and McKie 1996) although this may be simply a response to the cold climate. Chris Stringer and Clive Gamble (1993) write,

“Brain sizes in modern humans are related in a general way to both body size and the environmental temperature in which a population lives. These two factors are not independent of each other. Populations living in higher (cold) latitudes tend to be both larger brained and larger bodied … ”.

Several more recent skulls from China dated to 200,000 to 300,000 years old are very similar to Homo heidelbergensis or Archaic Homo sapiens from Europe and Africa (Stringer and McKie 1996). This may show some Homo heidelbergensis had migrated to China by this time. But interestingly the Levallois technology didn’t cross the Movius line and reach the region (“Technology”[Lower Palaeolithic]). It is difficult to believe a skill as useful as the Levallois could have been lost once it was discovered. The fact it didn’t get to this region indicates either that Homo heidelbergensis proper failed to get that far or, more likely, that yet again the technology and the genes were separate developments.

It would certainly make sense Homo heidelbergensis developed in an extremity of the human geographical range of the time. In fact it’s possible to interpret the fossil evidence as showing that about half a million years ago Homo heidelbergensis began evolving from Peking Man in the East Asian point of the human star. Their route out of the point would have been similar to the much more recent Mongol and Turkish expansion mentioned in “Indo-Europeans”. They expanded through the (possibly vacant) middle of the star and some of them eventually wandered into Africa. In Africa the immigrants mixed with the locals and developed the Levallois technique.

Then some of them, and especially the technology, moved back out. This pattern easily explains the separation between Neanderthal and modern human mtDNA lines mentioned earlier. We’ll return to mtDNA in “MtEve”[The Trees]. We know that migration into Africa has contributed to the human genes of that continent in more recent times (“Mythconceptions”[Modern Myths]) and generally once a migration route is established it is used many times. Sometimes gene movement has been in reverse. This route back into Africa would certainly be an old one. The defence mentioned in “The First Point”[Caucasus Population] it may go back to Homo erectus beginnings.

Australian Point

The Levallois technique certainly failed to reach Southeast Asia (“Technology”[Middle Paleolithic]). It also seems that Homo heidelbergensis didn’t genetically influence the population in that region very much either although over time their appearance did change a little.

As the defence said in “First Humans”[Expansion]Homo erectus held out in parts of Southeast Asia until at least as late as 100,000 years ago and probably even as recently as 30,000 (Tattersall and Schwartz 2000, and Jones 2001). Single origin supporters classify Southeast Asian Homo erectus as a distinct species and so they presumably would deny any influence from outside for the slight change in their appearance. The last of the Southeast Asian Homo erectus line are called “Solo Man” after the Solo River in Java. Stringer and McKie (1996) do concede “they showed many resemblances to” (both) “the Peking Man fossils and to their local Homo erectus predecessors”.

Could this perhaps have come about by the formation of a hybrid population? By this time their brain too had become larger and they have been variously classified as Homo sapiens, Homo erectus, a variety of Neanderthal or as a separate species Homo soloensis(Curtis, Swisher and Lewin 2001). They are now called Homo erectus again but they almost certainly survived in Southeast Asia long after modern humans or Homo sapiens had reached Australia. Some anthropologists, such as Dr. Susan Anton, have suggested the overlap in time between Homo erectus and modern humans “raises the possibility of gene flow between the two lines” (quoted in Wade 2001). We’ll look at this again in Part V (“Into Australia”[Kow Swamp]). Colin Tudge (1996) certainly sees nothing wrong with the idea our evolution is the product of a continuing series of movements of humans around the world forming hybrids. Examination of later Homo erectus mitochondrial DNA from Southeast Asia if possible could be very interesting.

Both of Them

A modern genetic distinction between the types in China is generally accepted (Jobling et al 2004). Interestingly, as the defence showed in “Pacific Population”[Mixing], the division explains Polynesian evolution. The boundary between the two eastern types is the Chin Ling Mountains at 34° north (Bellwood 1978). North of the mountains has always been dry and cold. And much of the region was tundra for long periods during the Pleistocene geological epoch (the ice age). At such times selection would have been severe. The area south of the mountains is moist and relatively warm. Human populations have easily become genetically and technologically adapted to it. Although the defence used different points on the human star to represent the two eastern populations the separation is as likely to be altitude as much as latitude. In other words the southern type was probably present along the warmer coast north of 34°. The coast actually stretches out far enough to combine Taiwan and Japan with the mainland at times of low sea level.

Presumably the two separate and distinctive eastern Homo erectus populations had developed locally from an earlier population migration. The evolution and expansion of Homo heidelbergensis later widened the difference between the populations of Northern and Southern China: the East Asian and Australian points of the human star.

Separation of ancient eastward human migration by the Hindu Kush and Himalayan Mountains could explain the different looking Homo erectus of Northern and Southern China. Perhaps the southern population came through the star’s Indian sub-point via the Iranian plateau. Certainly genetic movement into India has often come from the northwest. But the gene maps in Cavalli-Sforza et al (1994) tend to show that, at least between modern humans, very little if any genetic movement has occurred from India into Southeast Asia. Movement the other way, from Southeast Asia, has happened several times though (see for example map 5). Other evidence also shows that movement between India and Southeast Asia has more often been from east to west.

During any relatively moist and warm time Homo erectus could have moved east through Mongolia and then south into mainland Southeast Asia, the ancient Australian point. In other words Homo erectus expansion followed the arrows the defence used in map 12 to show that the Ganges / Brahmaputra delta has usually been impassable. Homo heidelbergensis genes were later added to the mix, especially to the northern population (if it didn’t in fact start its evolution there).

Recent evidence shows that the southern end of the Movius line was not as impermeable as was once thought though. Long after the initial Homo erectus expansion some elements of the hand-axe technology eventually moved from India into Southeast Asia. The jury has seen many examples of momentum carrying technology beyond genetic boundaries and so this transfer is hardly surprising. It probably occurred when the two Homo erectus branches eventually met up.

The world population between 200,000 and 100,000 years ago, in the period before mtEve’s descendants emerged from Africa, was distributed like this:

There were no people in any of the bottom three little sub-points and so we’ll leave them out. There were no people at all actually in Australia either but the population in Southeast Asia at the time can represent that point.

Ancient humans in each main point of the star looked different from each other and they were all changing.

By 100,000 years ago modern humans, Homo sapiens, had evolved in the star’s South and East African point. But before we move there we’ll visit the Northwest European point. There we’ll examine what have been called the last of these Archaic Homo sapiens, the Neanderthals. Although the two species’ mitochondrial DNA lines had parted merely half a million years ago we’ll look at what happened when Homo sapiens eventually met their cousins, Homo neanderthalensis.